1. Field of the Invention
This invention relates to an energy subtraction processing method and apparatus, wherein a subtraction process is performed on image signals representing radiation images of an object, which radiation images have been stored on stimulable phosphor sheets, and an image in which only an image pattern of a specific structure of the object, or the like, is thereby obtained.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
Also, energy subtraction processing techniques for radiation images have heretofore been known. With the energy subtraction processing techniques, at least two stimulable phosphor sheets are superposed one upon the other with an energy separation filter, which absorbs low energy components of radiation, intervening therebetween. The stimulable phosphor sheets are then exposed simultaneously to radiation carrying image information of an object, which is composed of a plurality of structures having different radiation energy absorption characteristics. In this manner, two radiation images of the object are stored respectively on the stimulable phosphor sheets. Thereafter, a subtraction process is performed on a first image signal, which represents a first radiation image having been stored on a first stimulable phosphor sheet located on the side close to the object, and a second image signal, which represents a second radiation image having been stored on a second stimulable phosphor sheet located on the side remote from the object. Specifically, the image signal components of the first image signal and the second image signal, which image signal components represent corresponding pixels in the first and second radiation images, are subtracted from each other. An energy subtraction image signal representing an energy subtraction image, in which an image pattern of only a specific structure or part of the object is illustrated, is thereby obtained.
In the energy subtraction processing techniques described above, such characteristics are utilized that a plurality of structures of an object, e.g. a bone and a soft tissue of a human body, exhibit different levels of radiation absorptivity with respect to radiation with different energy distributions. By the utilization of such characteristics, a plurality of radiation images are formed with several kinds of radiation with different energy distributions. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract an image of a specific structure, e.g. an image of a bone or an image of a soft tissue. The energy subtraction processing techniques are proposed in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037.
However, with the energy subtraction processing techniques described above, the radiation having passed through the first stimulable phosphor sheet, which is located on the side close to aradiation source, and having then passed through the energy separation filter impinges upon the second stimulable phosphor sheet, which is located on the side remote from the radiation source. Therefore, a comparatively small amount of radiation impinges upon the second stimulable phosphor sheet. As a result, the second image signal having been detected from the second stimulable phosphor sheet has a low signal-to-noise ratio. In cases where the subtraction process is performed by utilizing the second image signal having a low signal-to-noise ratio, an energy subtraction image signal having a low signal-to-noise ratio is obtained from the subtraction process. Accordingly, the problems occur in that an energy subtraction image having good image quality cannot be obtained. The problems become worse particularly in cases where an image recording operation is to be performed with a restricted dose of radiation.
The primary object of the present invention is to provide an energy subtraction processing method for performing a subtraction process on image signals representing radiation images of an object, which radiation images have been stored on stimulable phosphor sheets, wherein problems with regard to a low signal-to-noise ratio of an image signal having been detected from a stimulable phosphor sheet, which is located on the side remote from a radiation source, are eliminated, and an energy subtraction image having good image quality is capable of being obtained.
Another object of the present invention is to provide an apparatus for carrying out the energy subtraction processing method.
The present invention provides an energy subtraction processing method, wherein at least two stimulable phosphor sheets are superposed one upon the other with an energy separation filter, which absorbs low energy components of radiation, intervening therebetween and are then exposed simultaneously to radiation carrying image information of an object, which is composed of a plurality of structures having different radiation energy absorption characteristics, radiation images of the object are thus stored respectively on the stimulable phosphor sheets, and a subtraction process is performed on a first image signal, which has been detected by reading out a first radiation image having been stored on a first stimulable phosphor sheet located on the side close to the object, and a second image signal, which has been detected by reading out a second radiation image having been stored on a second stimulable phosphor sheet located on the side remote from the object, an energy subtraction image signal being thereby obtained, the method comprising the steps of:
i) utilizing a stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, as the second stimulable phosphor sheet,
ii) performing an addition process on a front surface side image signal, which has been detected by reading out the second radiation image from a front surface of the second stimulable phosphor sheet, and a back surface side image signal, which has been detected by reading out the second radiation image from a back surface of the second stimulable phosphor sheet, in order to obtain the second image signal, and
iii) performing the subtraction process on the first image signal and the thus obtained second image signal.
In the energy subtraction processing method in accordance with the present invention, the first stimulable phosphor sheet may be a stimulable phosphor sheet, which allows detection of an image signal from its one surface alone, and
the first image signal may be an image signal having been detected by reading out the first radiation image from the first stimulable phosphor sheet, which allows detection of an image signal from its one surface alone.
Alternatively, in the energy subtraction processing method in accordance with the present invention, the first stimulable phosphor sheet may be a stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, and
the first image signal may be an image signal having been detected by reading out the first radiation image from the front surface of the stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces.
As another alternative, in the energy subtraction processing method in accordance with the present invention, the first stimulable phosphor sheet may be a stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, and
the first image signal may be an image signal obtained from an addition process performed on a front surface side image signal having been detected by reading out the first radiation image from a front surface of the first stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, and a back surface side image signal having been detected by reading out the first radiation image from a back surface of the first stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces.
Also, in the energy subtraction processing method in accordance with the present invention, a thickness of a stimulable phosphor layer of the second stimulable phosphor sheet may be larger than the thickness of the stimulable phosphor layer of the first stimulable phosphor sheet.
The energy subtraction processing method in accordance with the present invention, wherein each of the first image signal and the second image signal is obtained from the addition process performed on the corresponding front surface side image signal and the corresponding back surface side image signal, may be modified such that at least three stimulable phosphor sheets, each of which allows detection of two image signals from its opposite surfaces, are utilized,
an operation for recording the radiation images of the object is performed by utilizing two stimulable phosphor sheets, which are among the at least three stimulable phosphor sheets, and
an operation for reading out a radiation image from a stimulable phosphor sheet, which is among the at least three stimulable phosphor sheets and on which a radiation image of an object has been recorded previously, is performed simultaneously with the operation for recording the radiation images of the object by utilizing the two stimulable phosphor sheets, which are among the at least three stimulable phosphor sheets.
Also, the energy subtraction processing method in accordance with the present invention, wherein each of the first image signal and the second image signal is obtained from the addition process performed on the corresponding front surface side image signal and the corresponding back surface side image signal, may be modified such that an addition ratio of the front surface side image signal in the addition process, which is performed on the front surface side image signal and the back surface side image signal having been detected from the first stimulable phosphor sheet, is higher than an addition ratio of the front surface side image signal in the addition process, which is performed on the front surface side image signal and the back surface side image signal having been detected from the second stimulable phosphor sheet, and the first image signal and the second image signal are thereby obtained.
The present invention also provides an apparatus for carrying out the energy subtraction processing method in accordance with the present invention. Specifically, the present invention also provides an energy subtraction processing apparatus, comprising:
i) an image recording section for simultaneously exposing at least two stimulable phosphor sheets to radiation carrying image information of an object in order to record radiation images of the object respectively on the stimulable phosphor sheets, the image recording section being constituted such that a first stimulable phosphor sheet is located on the side close to the object, a second stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, is located on the side remote from the object, and an energy separation filter, which absorbs low energy components of radiation, intervenes between the first stimulable phosphor sheet and the second stimulable phosphor sheet, and such that the first stimulable phosphor sheet, the second stimulable phosphor sheet, and the energy separation filter intervening therebetween are superposed one upon another,
ii) an image read-out section for reading out a first radiation image from the first stimulable phosphor sheet in order to obtain a first image signal, reading out a second radiation image from a front surface and a back surface of the second stimulable phosphor sheet in order to obtain a front surface side image signal and a back surface side image signal, and performing an addition process on the front surface side image signal and the back surface side image signal in order to obtain a second image signal, and
iii) an operation processing section for performing a subtraction process on the first image signal and the second image signal in order to obtain an energy subtraction image signal.
The energy subtraction processing apparatus in accordance with the present invention may be modified such that each of the at least two stimulable phosphor sheets is a stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, and
the image read-out section performs:
a) an operation for reading out the first radiation image from a front surface and a back surface of the first stimulable phosphor sheet in order to obtain a front surface side image signal and a back surface side image signal, and performing an addition process on the front surface side image signal and the back surface side image signal in order to obtain the first image signal, and
b) an operation for reading out the second radiation image from the front surface and the back surface of the second stimulable phosphor sheet in order to obtain the front surface side image signal and the back surface side image signal, and performing the addition process on the front surface side image signal and the back surface side image signal in order to obtain the second image signal.
The energy subtraction processing apparatus in accordance with the present invention, wherein each of the first image signal and the second image signal is obtained from the addition process performed on the corresponding front surface side image signal and the corresponding back surface side image signal, may be modified such that at least three stimulable phosphor sheets, each of which allows detection of two image signals from its opposite surfaces, are utilized,
an operation for recording the radiation images of the object is performed by utilizing two stimulable phosphor sheets, which are among the at least three stimulable phosphor sheets, and
an operation for reading out a radiation image from a stimulable phosphor sheet, which is among the at least three stimulable phosphor sheets and on which a radiation image of an object has been recorded previously, is performed simultaneously with the operation for recording the radiation images of the object by utilizing the two stimulable phosphor sheets, which are among the at least three stimulable phosphor sheets.
Also, the energy subtraction processing apparatus in accordance with the present invention, wherein each of the first image signal and the second image signal is obtained from the addition process performed on the corresponding front surface side image signal and the corresponding back surface side image signal, may be modified such that, in the image read-out section, an addition ratio of the front surface side image signal in the addition process, which is performed on the front surface side image signal and the back surface side image signal having been detected from the first stimulable phosphor sheet, is set at a value higher than an addition ratio of the front surface side image signal in the addition process, which is performed on the front surface side image signal and the back surface side image signal having been detected from the second stimulable phosphor sheet, and the first image signal and the second image signal are thereby obtained.
As a technique for reading out image information from a stimulable phosphor sheet, a technique for detecting two image signals from opposite surfaces of a stimulable phosphor sheet has heretofore been known, in which light emitted from the front surface of the stimulable phosphor sheet and light emitted from the back surface of the stimulable phosphor sheet are detected respectively, and two image signals are thereby detected from the opposite surfaces of the stimulable phosphor sheet. With the technique for detecting two image signals from opposite surfaces of a stimulable phosphor sheet, for example, a stimulable phosphor sheet comprising a transparent substrate and a stimulable phosphor layer overlaid on the front surface side of the transparent substrate is employed, and irradiation of stimulating rays is performed from the stimulable phosphor layer side of the stimulable phosphor sheet. When the stimulable phosphor sheet is exposed to the stimulating rays, light is emitted from each of the front surface side (i.e., the stimulable phosphor layer side) of the stimulable phosphor sheet and the back surface side (i.e., the transparent substrate side) of the stimulable phosphor sheet. The light emitted from the front surface side of the stimulable phosphor sheet and the light emitted from the back surface side of the stimulable phosphor sheet are respectively detected with photoelectric read-out means, which is located on the front surface side of the stimulable phosphor sheet, and photoelectric read-out means, which is located on the back surface side of the stimulable phosphor sheet. The technique for detecting two image signals from opposite surfaces of a stimulable phosphor sheet is disclosed in, for example, U.S. Pat. No. 4,346,295. In cases where the technique for detecting two image signals from opposite surfaces of a stimulable phosphor sheet is utilized, an addition process can be performed on the image signal components of the two image signals having been detected from the opposite surfaces of the stimulable phosphor sheet, which image signal components represent corresponding pixels on the front and back surfaces of the stimulable phosphor sheet. In this manner, the light collecting efficiency can be enhanced. Further, since noise components are uniformized, the signal-to-noise ratio of the obtained radiation image can be enhanced.
With the energy subtraction processing method and apparatus in accordance with the present invention, as described above, a stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, is employed as the second stimulable phosphor sheet, which is located on the side remote from the object. Also, the addition process is performed on the front surface side image signal, which has been detected from the front surface of the second stimulable phosphor sheet, and the back surface side image signal, which has been detected from the back surface of the second stimulable phosphor sheet. In this manner, the second image signal is obtained. Therefore, even if the dose of radiation impinging upon the second stimulable phosphor sheet is small, an image signal having a high signal-to-noise ratio can be obtained as the second image signal detected from the second stimulable phosphor sheet. As a result, an energy subtraction image having a high signal-to-noise ratio and good image quality can be obtained from the energy subtraction processing.
With the energy subtraction processing method and apparatus in accordance with the present invention, wherein a stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, is also employed as the first stimulable phosphor sheet, the energy subtraction processing can be performed by utilizing a single kind of the stimulable phosphor sheets, each of which allows detection of two image signals from its opposite surfaces. Therefore, the processing and management of stimulable phosphor sheets can be kept easier than when the two kinds of the stimulable phosphor sheets (i.e., the stimulable phosphor sheet, which allows detection of an image signal from its one surface alone, and the stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces) are utilized.
Also, with the energy subtraction processing method and apparatus in accordance with the present invention, wherein a stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces, is also employed as the first stimulable phosphor sheet, the first image signal may be constituted of only the image signal detected from the front surface of the first stimulable phosphor sheet, which allows detection of two image signals from its opposite surfaces. The image signal detected from the front surface of the first stimulable phosphor sheet is the one representing the radiation image formed with the radiation, in which the proportion of the low energy components is markedly high. Therefore, in such cases, the first image signal and the second image signal can be obtained such that they represent the radiation images having been formed respectively with two kinds of radiation with well-separated different energy levels. As a result, an energy subtraction image having image quality enhanced even further can be obtained from the energy subtraction processing.
With the energy subtraction processing method and apparatus in accordance with the present invention, the thickness of the stimulable phosphor layer of the second stimulable phosphor sheet may be larger than the thickness of the stimulable phosphor layer of the first stimulable phosphor sheet. In such cases, the thickness of the stimulable phosphor layer of the first stimulable phosphor sheet located on the side close to a radiation source becomes comparatively small, and the dose of radiation after having passed through the stimulable phosphor layer of the first stimulable phosphor sheet becomes large. Also, since the thickness of the stimulable phosphor layer of the second stimulable phosphor sheet located on the side remote from the radiation source becomes comparatively large, the stimulable phosphor layer of the second stimulable phosphor sheet can efficiently absorb the incident radiation. Accordingly, the second stimulable phosphor sheet can absorb more of the radiation, and the second image signal having a signal-to-noise ratio enhanced even further can be obtained.
Further, with the energy subtraction processing method and apparatus in accordance with the present invention, at least three stimulable phosphor sheets, each of which allows detection of two image signals from its opposite surfaces, may be utilized, and the operation for recording the radiation images of the object may be performed by utilizing two stimulable phosphor sheets, which are among the at least three stimulable phosphor sheets. Also, the operation for reading out a radiation image from a stimulable phosphor sheet, which is among the at least three stimulable phosphor sheets and on which a radiation image of an object has been recorded previously, may be performed simultaneously with the operation for recording the radiation images of the object by utilizing the two stimulable phosphor sheets, which are among the at least three stimulable phosphor sheets. In such cases, since the operation for recording the radiation images of the object on the two stimulable phosphor sheets and the operation for reading out the radiation image having been recorded previously on a stimulable phosphor sheet are performed simultaneously, the time required to perform the entire processing can be kept short, and the intervals between successive image recording operations can be kept small.
Furthermore, with the energy subtraction processing method and apparatus in accordance with the present invention, the addition ratio of the front surface side image signal in the addition process, which is performed on the front surface side image signal and the back surface side image signal having been detected from the first stimulable phosphor sheet, may be set at a value higher than the addition ratio of the front surface side image signal in the addition process, which is performed on the front surface side image signal and the back surface side image signal having been detected from the second stimulable phosphor sheet, and the first image signal and the second image signal may thereby be obtained. As will be described later, the ratio of the intensity of the light, which is emitted from the front surface of the first stimulable phosphor sheet, to the intensity of the light, which is emitted from the back surface of the first stimulable phosphor sheet, is higher than the ratio of the intensity of the light, which is emitted from the front surface of the second stimulable phosphor sheet, to the intensity of the light, which is emitted from the back surface of the second stimulable phosphor sheet. Therefore, with the energy subtraction processing method and apparatus in accordance with the present invention, wherein the addition ratios are set in the manner described above, the first image signal and the second image signal, which have signal-to-noise ratios enhanced even further, can be obtained.